The present invention relates generally to a magnetic memory device in which at least one of the conductors includes an asymmetric cladding. More specifically, the present invention relates to a magnetic memory device in which at least one of the conductors includes an asymmetric cladding that is recessed to minimize undesirable effects to a switching characteristics of a data layer of a magnetic memory cell while providing for switching field enhancement.
Magnetic Random Access Memory (MRAM) is an emerging technology that can provide an alternative to traditional data storage or memory technologies. MRAM has desirable properties such as fast access times like DRAM and non-volatile data retention like hard disc drives. MRAM stores a bit of data (i.e. information) as an alterable orientation of magnetization in a patterned thin film magnetic element that is referred to as a data layer, a storage layer, a free layer, or a data film. The data layer is designed so that it has two stable and distinct magnetic states that define a binary one (xe2x80x9c1xe2x80x9d) and a binary zero (xe2x80x9c0xe2x80x9d). Although the bit of data is stored in the data layer, many layers of carefully controlled magnetic and dielectric thin film materials are required to form a complete magnetic memory element. One prominent form of magnetic memory element is a spin tunneling device. The physics of spin tunneling is complex and good literature exists on this subject.
In FIG. 1a, a prior MRAM memory element 101 includes a data layer 102 and a reference layer 104 that are separated by a thin barrier layer 106. Typically the barrier layer 106 has a thickness that is less than about 2.0 nm. The memory element 101 has a width W and a length L and a ratio of the width W to the length L defines an aspect ratio (i.e. aspect ratio=W÷L). In a tunneling magnetoresistance memory (TMR) the barrier layer 106 is an electrically non-conductive dielectric material such as aluminum oxide (Al2O3), for example. Whereas, in a giant magnetoresistance memory (GMR) the barrier layer 106 is a thin layer of conductive material such as copper (Cu), for example. The reference layer 104 has a pinned orientation of magnetization 108, that is, the pinned orientation of magnetization 108 is fixed in a predetermined direction and does not rotate in response to an external magnetic field. In contrast the data layer 102 has an alterable orientation of magnetization 103 that can rotate between two orientations in response to an external magnetic field. The alterable orientation of magnetization 103 is typically aligned with an easy axis E of the data layer 102.
In FIG. 1b, when the pinned orientation of magnetization 108 and the alterable orientation of magnetization 103 point in the same direction (i.e. they are parallel to each other) the data layer 102 stores a binary one (xe2x80x9c1xe2x80x9d). On the other hand, when the pinned orientation of magnetization 108 and the alterable orientation of magnetizations 103 point in opposite directions (i.e. they are anti-parallel to each other) the data layer 102 stores a binary zero (xe2x80x9c0xe2x80x9d).
In FIG. 2a, the prior memory element 101 is typically positioned at an intersection of two orthogonal conductors 105 and 107. For instance, the conductor 105 can be a word line and the conductor 107 can be a bit line. Collectively, the conductors (105, 107) can be called write lines. A bit of data is written to the memory element 101 by generating two magnetic fields Hx and Hy that are in turn generated by currents Iy and Ix flowing in the conductors 107 and 105 respectively. The magnetic fields Hx and Hy cooperatively interact with the data layer 102 to rotate the alterable orientation of magnetization 103 from its current orientation to a new orientation. Therefore, if the current orientation is parallel (i.e. a positive X-direction on a x-axis X) with the pinned orientation of magnetization 108 such that a binary xe2x80x9c1xe2x80x9d is stored in the data layer 102, then the magnetic fields Hx and Hy will rotate the alterable orientation of magnetization 103 to an anti-parallel orientation (i.e. a negative X-direction on the x-axis X) such that a binary xe2x80x9c0xe2x80x9d is stored in the data layer 102.
In FIG. 2a, the alterable orientation of magnetization 103 is illustrated in the process of rotating from the positive X-direction to the negative X-direction. Both of those directions are aligned with the easy axis E. However, during the rotation, the alterable orientation of magnetization 103 will be temporarily aligned with a hard axis H that is aligned with a positive Y-direction and a negative Y-direction of a y-axis Y.
In FIG. 2b, the prior memory element 101 is positioned in an array 201 of similar memory elements 101 that are also positioned at an intersection of a plurality of the conductors 107 and 105 that are arranged in rows and columns. For purposes of illustration, in FIG. 2b, the conductors 107 are bit lines and the conductors 105 are word lines. A bit of data is written to a selected one of the memory elements 101 that is positioned at an intersection of a word and bit line by passing the currents Iy and Ix through the word and bit lines. During a normal write operation, the selected memory element 101 will be written to only if the combined magnetic fields Hx and Hy are of a sufficient magnitude to switch (i.e. rotate) the alterable orientation of magnetization of the memory element 101.
In FIG. 3a, when the alterable orientation of magnetization 103 is aligned with the easy axis E, the prior data layer 102 will have magnetic charges, denoted as a plus sign+and a minus signxe2x88x92, and those magnetic charges (+, xe2x88x92) generate a demagnetization field HDE. The demagnetization field HDE facilitates switching of the data layer 102 by reducing a magnitude of the combined magnetic fields (Hx, Hy) that are required to rotate the alterable orientation of magnetization 103. Essentially, the amount of energy required to rotate the alterable orientation of magnetization 103 is reduced.
Similarly, in FIG. 3b, when the alterable orientation of magnetization 103 is in a partially rotated position that is parallel to the hard axis H, another demagnetization field HDh is generated by magnetic charges (+, xe2x88x92). Those magnetic charges oppose further rotation of the alterable orientation of magnetization 103.
The switching characteristics of the data layer 102 are determined in part by the magnitudes of the demagnetization fields (HDE, HDh). Preferably, the magnitude of the demagnetization field HDE is sufficient to reduce the amount of energy required to initiate rotation of the alterable orientation of magnetization 103 and the magnitude of the demagnetization field HDh is sufficient to slightly resist further rotation of the alterable orientation of magnetization 103 so that as the alterable orientation of magnetization 103 passes through the hard axis H, the data layer 102 does not immediately switch (i.e from a logic xe2x80x9c1xe2x80x9d to a logic xe2x80x9c0xe2x80x9d).
One of the disadvantages of prior MRAM designs is that the currents (Iy, Ix) that are required to generate the combined magnetic fields (Hx, Hy) are too high. High current is undesirable for several reasons. First, high currents increase power consumption which is undesirable in portable electronics or battery powered electronics. Second, high currents can result in increase waste heat generation which can require fans or other cooling devices to efficiently dissipate the waste heat. Those cooling devices add to the cost, weight, and power drain in battery operated devices. Third, larger drive circuits are required to source those high currents and the larger drive circuits reduce an amount of die area available for memory or other critical circuits in a memory device. Finally, the conductors that carry the current can fail due to electromigration caused by a high current density in the conductors.
Prior methods for reducing the currents (Iy, Ix) include cladding the conductors (105, 107) with a soft magnetic material as illustrated in FIG. 4. A cladding 112 that covers an entirety of three sides of a conductor 120 and includes poles p that are positioned flush with a surface of the conductor 120 and adjacent to the data layer 102. The cladding 112 enhances the available magnetic field and creates a closed magnetic path 110 in a direction along the x-axis X. The closed magnetic path provides flux closure of the magnetic fields (Hx, Hy) and efficiently couples those fields with the data layer 102.
Additionally, the closed magnetic path 110 increases an effective length of the data layer 102 thereby increasing a shape anisotropy of the data layer 102. A larger shape anisotropy increases the magnetic stability of the data layer 102. Typically, to achieve the desired magnetic stability via shape anisotropy, the data layer 102 is made longer in the direction of the easy axis E such that the memory cell 101 is longer in the width W dimension than it is in the length L dimension. Although the increase in effective length further increases the magnetic stability of the data layer 102 it also has the disadvantage of making it harder to switch the data layer 102. As a result, more current is required to effectuate switching the data layer 102. Because the data layer 102 has already been made physically longer in the direction of the easy axis E, that physical length is made even longer by the addition of the effective length created by the closed magnetic path 110. Consequently, the use of the cladding 112 exacerbates the switching current requirements even though the cladding 112 has the effect of focusing the available magnetic field at the data layer 102.
Moreover, the desired increase in shape anisotropy comes at the expense of increased memory cell size because increasing the physical length of the data layer 102 also increases the area occupied by the memory cell 101. As a result, aspect ratios in a range of about 2.0 to about 3.0 or more are common. Consequently, areal density is reduced because the memory cell 101 occupies a larger area. Ideally, the aspect ratio should be as close to to 1.0 as possible so that areal density can be increased.
In FIG. 5c an array 203 includes the prior memory elements 101 positioned at an intersection between a pair of cladded conductors (105c, 107c). In FIG. 5a, a cross-sectional view along the X-axis X of FIG. 5c, illustrates the conductor 107c having a cladding 109 that covers an entirety of three sides of the conductor 107c and includes poles 109p that are positioned flush with a surface of the conductor 107c and are positioned adjacent to the data layer 102.
In FIG. 6a, one disadvantage of the structure of FIG. 5a is that the poles 109p generate magnetic charges (+, xe2x88x92) of opposite polarity to those generated by the data layer 102. The magnetic charges (+, xe2x88x92) generated by the poles 109p significantly reduce or cancel the demagnetization field Hxe2x80x2DE so that more energy (i.e. a stronger magnetic field Hx) is required to rotate the alterable orientation of magnetization 103. As a result, more current Iy must be supplied to generate a higher magnitude of the magnetic field Hx. Therefore, the cladding 109 has a detrimental effect on one component of the switching characteristic of the data layer 102. Moreover, the cladding 109 increases the effective length of the data layer 102 as was described above. Consequently, the current Iy must be further increased to effectuate switching the data layer 102. The closed magnetic path generated by the cladding 109 creates a low reluctance path through the data layer 102 resulting in a strong coupling of the magnetic field with the data layer 102 and the effects of the aforementioned demagnetization field Hxe2x80x2DE and the increase in the effective length of the data layer 102 are exacerbated by that strong coupling.
In FIG. 5b, a cross-sectional view along the Y-axis Y of FIG. 5c, illustrates the conductor 105c having a cladding 111 that covers an entirety of three sides of the conductor 105c and includes poles 111p that are positioned flush with a surface of the conductor 105c and are positioned adjacent to the data layer 102.
In FIG. 6b, one disadvantage of the structure of FIG. 5b is that the poles 111p generate magnetic charges (+, xe2x88x92) of opposite polarity to those generated by the data layer 102. Those magnetic charges reduce a coercivity of the data layer 102 with a resulting reduction or cancellation of the demagnetization field Hxe2x80x2Dh. As a result, there is little or no resistance to rotation of the alterable orientation of magnetization 103 as it passes through the through the hard axis H. Therefore, the cladding 111 has a detrimental effect on another component of the switching characteristic of the data layer 102. The effects of the demagnetization field Hxe2x80x2Dh are also exacerbated by a strong coupling of the magnetic field with the data layer 102 due to the cladding 111.
Consequently, there exists a need for a cladding structure for one or more conductors of a magnetic memory cell that provides for switching field enhancement without the detrimental effects to a switching characteristics of a data layer of the magnetic memory cell. There is a need for a cladding structure that increases the reluctance of a magnetic path so that magnetic coupling is reduced. There is also a need for a cladding structure that allows for a reduction in an aspect ratio of the magnetic memory cell so that areal density is increased.
The present invention address the aforementioned problems created by the prior cladded conductors by asymmetrically cladding one or more of the conductors that cross a magnetic memory cell. A magnetic memory device includes a magnetic field sensitive memory cell and a first conductor that crosses the memory cell in a length direction and a second conductor that crosses the memory cell in a width direction. The first conductor includes a first cladding that covers a top surface and a portion of two side surfaces thereof. The cladding includes a first pair of poles that are positioned adjacent to the two side surfaces and are recessed along the two side surfaces by a first offset distance.
In one embodiment, the second conductor also includes a second cladding that covers the top surface and a portion of the two side surfaces thereof. The second cladding includes a second pair of poles that are positioned adjacent to the two side surfaces and are recessed along the two side surfaces by a second offset distance. The first and second offset distances need not be equal to each other so that the first and second poles are not equidistantly spaced apart from each other relative to a data layer of the memory cell or some other reference point on the memory cell.
Either one or both of the first and second conductors can be laterally displaced relative to their respective crossing directions such that the conductor is not symmetrically centered with the memory cell. The lateral displacement can be used to change a point within the data layer in which nucleation of switching occurs with a resulting change in a switching characteristic of the memory cell.
The aforementioned problems caused by a strong magnetic coupling between the magnetic field generated by the cladded conductor and the data layer are reduced by the recessed poles because the reluctance of the magnetic path is increased. The increased effective length of the data layer along the easy axis is also reduced by the recessed poles because the increased reluctance operates to reduce coupling of the magnetic field with the data layer.
The recessed poles reduce the cancellation effect of the magnetic charges generated by the poles such that demagnetization fields along an easy axis and a hard axis of the data layer are reduced or eliminated and a desirable switching characteristic of the data layer is obtained while also enhancing a switching field generated by current flowing in the first and second conductors.
By asymmetrically cladding one or both of the first and second conductors, an aspect ratio of the memory cell can be reduced over prior memory cells with cladded conductors.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.